1. Field of the Invention
The invention relates to low-monomer containing, NCO-containing prepolymers (hereinafter NCO prepolymers) based on 1,4-diisocyanato-2,2,6-trimethylcyclohexane (TMCDI), to a process for their preparation, and to their use.
2. Discussion of the Background
Aliphatic and cycloaliphatic diisocyanates are widely employed in high-grade light-and weather-stable polyurethane coatings. For many applications, such as for prepolymer formation, it is advantageous if the two isocyanate groups have different reactivities. This so-called selectivity reduces the proportion of unwanted monomers, and the resulting products have lower viscosities and improved technical processability.
Hexamethylene diisocyanate (HDI) exhibits no selectivity whatsoever and so the corresponding NCO prepolymers have a high proportion of unwanted, toxicologically objectionable monomers. Isophorone diisocyanate (IPDI), on the other hand, is selective owing to the different steric environment of the two isocyanate groups, but prepolymerization under identical conditions leads to relatively low monomer contents. In both cases, the monomer content may be lowered by means of a technically complex short-path (flash) distillation.
The object was to find NCO prepolymers having a very low monomer content and being suitable, moreover, for use directly, without a distillation process for the purpose of removing the monomers.
It has surprisingly been found that 1,4-diisocyanato-2,2,6-trimethylcyclohexane (TMCDI), a readily available cycloaliphatic diisocyanate (J. Appl. Polym. Sci. 1994, 54(2), 207-218; EP 0945427 A1) is particularly suitable for use in NCO prepolymers. Without any complex short-path distillation, the residual monomer content is distinctly lower than in the case of prepolymers based on IPDI.
The invention provides for a low-monomer containing, NCO-containing prepolymer based on 1,4-diisocyanato-2,2,6-trimethylcyclohexane (TMCDI) and polyol, having a residual monomer content of less than 1.0% by weight, obtainable by reacting a diisocyanate component comprising 1,4-diisocyanato-2,2,6-trimethylcyclohexane (TMCDI) and at least one at least difunctional polyol in an NCO/OH ratio of 2-1.5:1 in the presence of at least one catalyst at 20-100xc2x0 C.
The invention likewise provides a process for preparing NCO-containing prepolymers based on TMCDI and polyols, having a residual monomer content of less than 1.0% by weight, by reacting TMCDI and at least one polyol in an NCO/OH ratio of 2-1.5:1 in the presence of at least one catalyst at 20-100xc2x0 C.